Wide Crosses
- To: Multiple recipients of list <i*@rt66.com>
- Subject: Wide Crosses
- From: S* M* <7*@CompuServe.COM>
- Date: Sat, 15 Mar 1997 10:08:52 -0700 (MST)
Walter Moores asked:
: Sharon, where are you when we need you the most?
Taking advantage of the extremely rare opportunity to spend a week with my
grandmother's cousin -- devoting more time to family genealogy than iris
pedigrees..... Bet you thought NOTHING in my life could be more important than
iris!
: Please tell us
: what a 'wide cross' is !!!
It means different things to different people working with different types of
iris. I tend to think of it as a continuum -- a relative description, rather
than a technical term. In a nutshell, I haven't seen any definition I'd really
disagree with. To me, it's really a matter of context.
To those who normally line-breed and work strictly within a fertile family, an
outcross is relatively "wide" because it clearly has a greater disparity among
its genes.
A more common usage labels a cross between plants with nonhomologous chromosomes
as "wide". These crosses, such as one between widely separated species,
"shouldn't" take.
There's a middle ground where it's hard to draw a clear line between "normal"
and "wide" crosses. The world of arils and arilbreds serves as a good example
because it is such a complex one. For purposes of illustration I'll lump the
different types into five groups with each # wider than the one before it, not
as wide as the one following it:
1. A cross between two members of the same fertile family (such as 1/2-bred
X 1/2-bred or onco X onco) isn't a wide one in my mind, even if the two have no
identifiable common ancestors, because a reasonable number of fertile offspring
are expected. But some WOULD consider a regeliabred X oncobred cross "wide",
and it does meet the criteria for having some nonhomologous chromosomes.
2. A cross between two closely related families, such as an onco crossed
with a diploid regelia. Fewer seeds, fewer seedlings, less fertility in the
offspring as compared to crosses within either family.
3. A cross between two quite different fertile families (such as TB X
1/2-bred; 1/2-bred X aril) that involves nonhomologous chromosomes, yet tend to
produce a reasonable number of seedlings -- albeit relatively infertile ones.
These are made quite routinely.
4. A cross between a member of one of the fertile families and a relatively
infertile hybrid (1/4-bred X 1/2-bred or 1/2-bred X 3/4-bred). Enough
nonhomologous chromosomes to present a challenge, enough compatible ones to
provide an opportunity.
5. A cross (like tetraploid TB X diploid aril) that involves not only
individual nonhomologous chromosomes, but vastly mismatched numbers. The
expectation is few takes, few seeds, poor germination. And rarely any
fertility. BUT there's always the chance of getting another IB-MAC or WELCOME
REWARD, so I keep trying.
"Wide" is certainly an apt description of #5, but I find it hard to draw the
line. When making crosses, it would be tempting to lump #'s 1, 2 & 3 together
as relatively easy, #'s 4 & 5 together as "wide". When processing seeds, I'd
group #'s 1 & 2 as appropriate for natural germination, #'s 3, 4, & 5 and needed
special treatment.
My hunch is that everyone who works with arils and arilbreds draws the line a
bit differently. You expected, perhaps, a simple answer?
Sharon McAllister (73372.1745@compuserve.com)
who doesn't have time to write a short answer at the moment
